Semiconductor wafers used in the manufacture of integrated circuits are often marked with identifying information to facilitate in-process tracking during their production. The identifying marks, known as scribe marks, typically comprise a series of characters, bar codes, or other two-dimensional codes, each of which is formed from small circular depressions or "pits" in the substrate. The pits are typically formed by using a laser to melt a small area of the substrate, the melted material being removed in subsequent etching steps. Because integrated circuits should not be formed on the scribe marks, they are typically positioned near the unused outer edge of the wafer by the primary fiducial to maximize the wafer surface area available for forming integrated circuits.
Using a human operator to manually read the marks directly from the wafers is labor intensive and susceptible to error. Moreover, the marks are not always accessible to the operator during wafer processing. Using an imaging system that presents an enlarged image of the mark to the operator increases the accuracy of the observation, but the process is still labor intensive and susceptible to error because of poor image quality. Efficient wafer fabrication requires that the marks be reliably machine-readable by automated process equipment. Some marks, such as bar codes, are readable essentially only by a machine.
Automated reading of a mark is a three-step process. A camera typically forms an image of the scribe mark, and the image is converted into a computer-compatible digital format. The digitized image is then interpreted, for example, by optical character recognition software to determine letters, numbers, bar codes, or other symbols in the digitized image. For the mark to be interpreted by the software, the digitized image must be relatively clear, i.e., there must be adequate contrast between the background and the image of the mark.
It has been difficult to form a clear image of the scribe marks for several reasons. Because typical scribe marks comprise a group of relatively shallow depressions in the substrate, the marks are of the same color as the substrate background and are, therefore, hard to differentiate. The pits of shallow or "soft" marks, which are used to reduce contamination during wafer processing, can have a depth of only one micron. Moreover, the substrates are typically highly polished and, therefore, reflect into the camera a large amount of light that tends to obscure the mark. Light from the processed areas on the water surface around the pits of the scribe mark is process "noise" that tends to obscure the light "signal" from the scribe mark.
During wafer fabrication, layers are formed on the wafers using a variety of materials having a wide range of reflectivity and other optical properties. Illumination levels required to achieve an image having adequate contrast between the background and the mark can vary over three orders of magnitude, thereby requiring frequent gain adjustments to the charge-coupled device ("CCD") camera. The automatic gain control on CCD cameras reacts too slowly to such a wide range of brightnesses and, if used, unacceptably delays processing. Manual gain adjustments or sequenced changes in the illumination level can be used to compensate for the variation in image intensity, but such methods also slow production. Moreover, manual adjustments require an operator to continuously monitor the scribe mark reader.
The optical properties of the wafer surfaces vary not only from wafer to wafer but also across the surface of an individual wafer. Wafers can exhibit an "edge bead," i.e., an abrupt change in contrast and reflectivity of the surface as a result of a layer, such as a metal or oxide coating, not extending to the outer periphery of the wafer. The edge bead can appear across the scribe mark itself, producing artifacts in an image formed of the mark. Imperfect forming or etching of the layers can also cause thickness variations in the layers, especially at the edges of the wafers where the thickness variations are observable as annular rings. The coloration and nonuniform reflectivity of such rings, located at the edge of the wafer where the scribe marks are also located, interfere with reading the scribe marks by producing artifacts in the image. Furthermore, materials used to treat or coat wafers, particularly photoresist, can accumulate in the depressions of the scribe marks, further obscuring the mark by affecting the optical properties of the substrate surface and scribe marks.
Because the problems described above make it difficult to form a clear image of the scribe marks, the optical character recognition software often cannot interpret the poor quality image it receives. Scribe mark readers typically require, therefore, frequent intervention from human operators, either to adjust the optical elements to improve the image or to manually read the mark or significant investment by semiconductor manufacturers to improve or add additional process steps to improve the scribe readability.